[TowerTalk] PVC & Losses

[James R. Duffey]

Sig - In reply to your question:

"Wonder if anyone could comment on the use of 4" OD PVC Tubing as a form for
a loading coil that will be used in an above ground shortened radial system
on each element of a 4-SQUARE?  Should I hunt around for Polyethylene or is
the PVC OK for this type application."

The PVC should be OK, particularly in the fairly large diameter you propose
using. The losses at HF for PVC are not all that much worse than other
dielectrics often reccomended for RF use. However PVC does have a lower
softening point than most other plastics, which can lead to problems if
losses are excessive.

PVC got its bad rap from a picture in QST 20 or more years ago. It showed a
PVC former that had been used for a loading coil on a 160 M antenna that had
softened and split open due to excessive losses. This is a pathoglogical
case however, the coil, long and thin, had a low Q and high losses. It was
also wound with small diameter wire which further compounds losses.

In one of the recent Antenna Compendiums there was an article on mobile
antennas . The author compared two inductors; one wound on a PVC former and
one air wound. The PVC former was only slightly lower and the author
concluded that it was acceptable for use as loading coils.

I don't know how many radials you are going to load, but the total power
dissipated gets divided by the number of radials which further reduces the
losses in any indiviual coil.

The best way to evaluate the PVC former in this use is to use it in the
application and see if it gets hot. Build a few radials, run them with the
power you are going to run for a whilw and see if they get hot. Don't touch
them with power applied. :^). If they don't get too hot you should be ok.

Some recommend using a microwave oven to determine losses in materials.
However the losses at microwave frequencies can be different than those at
HF, so you may as well do the tests at HF anyway.

I hope that this helps. - Dr. Megacycle KK6MC/5


James R. Duffey KK6MC/5
30 Casa Loma Road
Cedar Crest, NM 87008

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